The Nasa probe Stardust has had a dramatic encounter with Comet Wild-2, passing just 240 km away from the mountainous ball of ice, rock and dust.

The craft sent back startling images of the object and grabbed particles streaming away from its nucleus for return to Earth in two years' time.

The flyby occurred in deep space 389 million km from Earth at 1944 GMT.

Scientists say the probe's data will yield valuable information on the early history of the Solar System.

"The comet co-operated better than we could have expected and the spacecraft worked wonderfully well," said Don Brownlee, of the University of Washington and the principal investigator for the Stardust mission.

"We've collected dust from a comet and we're bringing it home for analysis in laboratories all over the world."

Big jets

Comet Wild-2 is probably 5.4 km (3.3 miles) across. It sailed past the probe at a relative speed of 21,960 km per hour (13,650 miles per hour).

A mass of data collected in the course of the encounter was due to be fed back over more than 30 hours, including about 70 images.

Stardust's mission: How Nasa's probe collected comet dust and will return it to Earth

"These images are better than we had hoped for in our wildest dreams," said Ray Newburn, a co-investigator for Stardust. "They will help us better understand the mechanisms that drive conditions on comets."

The first pictures showed a roughly spherical comet nucleus that was heavily pitted. Four or five major jets of material could also be seen bursting from the object.

The spacecraft's dust flux instrument suggested Stardust flew straight through two of these big jets.

"We expected the flux to increase as we approached and then decrease, but we also saw big bursts of particles which would have terrified us had we known about it in advance," said Tom Duxbury, Stardust project manager.

Old and cold

The surface pockmarks, some of them cavernous in size, have already drawn comment from researchers and journalists.

Such features have never before been seen on a close-up image of a comet nucleus and raise the intriguing possibility that these "dirty snowballs", just like planets, moons and asteroids, retain the scars of past impacts.

But Dr Brownlee was sceptical. He thought they were "sinkholes", which formed as the comet lost material.

"I don't think they are actually impact craters; I think they are erosion features for the most part, where the more volatile material has formed the huge jets and the atmospheric coma that we see around the comet."

Dust from Wild-2 was scooped up by a robotic collector filled with aerogel - a very low-density glass - and stowed inside a sample-return capsule.

This will be delivered to Earth on 15 January, 2006; the capsule making a soft landing at the US Air Force Utah Test and Training Range.

'Time capsule'

If the return works, the particles would represent the second robotic retrieval of extraterrestrial material since 1976, when the unmanned Soviet Luna 24 mission brought back samples of rock and soil from the Moon.

Nasa's Genesis spacecraft should be the first since then come September, when it returns samples of the solar wind it has collected in space.

Scientists believe in-depth terrestrial analysis of the Wild-2 samples will reveal much about comets and the earliest history of the Solar System.

Chemical and physical information locked within the cometary particles could be the record of the formation of the planets and the materials from which they were made.

"Comets are made of ice and are very cold and have been very cold since they were formed," said Dr Simon Green, an Open University, UK, scientist on the Stardust project.

"That protects the material of which they were made from any process of heating, so they haven't been changed since they were formed, right at the beginning of the formation of the Solar System.

"So we can have almost a little time capsule of what things were like 4.5 billion years ago."